Method of burning coke from heater tubes



Patented Oct. 13, 1936 PATENT OFFICE METHOD OF BURNING COKE FROM ,HEATERTUBES Chester B. McAllister, Los Angeles, Calif.. ,assignor to TheTexas'Company, New York, N. Y., a

corporation of Delaware No Drawing. Application May 15, 1935,

Serial No. 21,686

2 Claims. (Cl. 196-69) This invention relates to a method of removingcoke r carbonaceous deposits from the inner walls of tubular heaters inoil distilling or cracking systems, which are externally fired and whichcarry a stream of petroleum or a fraction thereof to be heated orcracked and during which coke is formed and deposited on the tube walls.

The invention is particularly concerned with the removal of cokedeposited in oil heater tubes as the result of the cracking of heavyoils therein, by subjecting the coke to the heat of a localized flame inthe presence of an oxygen-containing gas. It has been found that thecoke deposited in oil heater tubes as the result of cracking light oilsof the character of gas oil may successfully be burned from the tubesafter the ordinary steaming of the tubes to purge them of oil vapors, ascommonly practiced in the industry when shutting down a cracking unit.-This method 0 of burningthe coke deposit from the tubes may be carriedout in accordance with the teaching of Davis Patent No. 1,901,803.However, where heavy oils are subjected to heating and cracking intubular heaters,it has been found'that mere purging of the tubes withsteam is insufficient to condition the coke so that it may successfullybe removed by the burning operationl In such cases the coke has beenfound to contain substantial amounts of the heavy oil or tars, and whenthe burning operation is attempted on such deposits of coke excessiveamounts of oxygen are consumed and local overheating of the tubes mayresult.

I have found that it is possible to so condition the coke deposited asthe result of heating and cracking heavy oils as to enable it to readilybe burned out of the tubes by means of a localized flame in the presenceof oxygen-containing gas,

such as air or air plus oxygen, as disclosed in the aforesaid Davispatent. V One method of accomplishing these ends consists essentially inthe following steps:

(a) Passing a relatively clean light oil, such as gas oil, through thetubes after shutting off the supply of heavy charge oil whilemaintaining the firing of the furnace;

(b) Discontinuing the supply of gas oil to the tubes and cutting out thefires in the heater;

(0) Introducing steam into the tubes while still hot and steaming untilsubstantially no oil vapors are evolved; 1

(d) Removing headers, return bends or the like and burning the coke fromthe tubes by the method mentioned above.

An alternative method of accomplishing the desired results may consistessentially in the following steps:

(1) Cutting out the fires in the furnace and discontinuing the charge ofheavy oil to the 5- tubes;

(2) Steaming the tubes while hot until substantially no further oilvapors are evolved;

(3) .Discontinuing the steaming and passing hot Water through the tubes;10

(4) Cutting off the water supply; removing headers, return bends, etc.,and burning the coke from the tubes in accordance with the methoddescribed.

Coke deposits resulting from the cracking of 15 heavy crude, reducedcrude or residual oils have been found to contain from 3-7% of oil byextracting the same with benzol. When such large proportions of heavyoil remain in the coke, the deposit is removed by burning at a very slow2 rate with the generation of a great deal of heat, causing the tube tobecome overheated. Such oily coke is also rather gummy and does notpermit ready passage of the burner head. I have discovered that if theoil content of the 5 coke is reduced to about 2% or less, preferably notmore than 1%%, the burning of the 'coke from the tubes may successfulybe practiced in the presence of oxygen-containing gas. The removal ofthe excess of heavy oil from the coke 30 deposit may readily beaccomplished by the practice of my invention.

In order to more fully describe the use of my invention, the followingspecific examples are given: 5

The tubular oil heater had been in operation in the cracking of fuel oilor a heavy crude. After substantial deposits of coke had been formed inthe tubes, the heavy charge stock was discontinued and a clean gas oilwas passed through 40 the tubes for a period of about 3 hours beforecutting the fires in the furnace. The fires were discontinued andimmediately thereafter the tubes were steamed for a period of about 3hours. In case the coke deposit was particularly heavy, 45 the steamingwas continued for a longer period, up to about 8 hours. The steamingordinarily was carried on until substantially no further oil vapors wereevolved. Then the return bends or junction boxes were removed from thetubes and 50 a fuel gas burner slowly passed through each tube,whilesupplying a current of oxygen or air through the tubes. Thevelocity of the oxygencontaining gas was sufi'icient to break out andforce out the coke, which was continuously dis- 55 lodged anddisintegrated by the flame of the burner. The coke that was dislodgedand disintegrated from the tube walls by the action of the flame and theair or oxygen jet was continuously blown out of the end of the tubeahead of the burner. The proportions of air, fuel gas and oxygen wereregulated to suit the particular conditions encountered, as evidenced bythe appearance of the material leaving the end of the tube and byobservation of the outside of.

the tube walls. I

It is not known whether the steam in the conditioning step merely driesout the coke or wheth' er there is some additional beneficial effect. Iti is possible that moisture is deposited in the pores of the coke duringsteaming, which may greatlyaccelerate the spalling off or disintegrationof the coke during the heating and burning step. However, it is observedin the operation of the process that a portion of the coke is ejectedfrom the tubes in the form of fine incandescent particles, and that alarger portion is not incandescent and consists of much larger particlesof the order of small chips or chunks.

In the alternative method of operation, the fires were cut and thecharge of heavy oil continued for 20 to 30 minutes, after which it wasshut off. As soon as the pressure in the heater dropped sufiiciently topermit introduction of high pressure'steam (150-200 lbs. per sq. in.)from the plant mains, the steam was cut into the heater and the tubessteamed for a period of 3 to 4 hours, or until no further oil vaporswere discharged. Then the steam was shut oif and water at a temperatureof about ISO-200 F. was pumped through the heater for a period of about30 to minutes at a rate of about gallons per minute. The water supplywas shut off and the return bends and the like removed. The tubes werethen in condition to be burned out in accordance withthev procedurealready described.

By either of the methods described above it has been found possible toreduce the heavy oil content of the coke deposited in the tubes to lessthan 2%, as determined by extraction with b enzol. As a result, theheavy coke deposit formed in the cracking of; heavy oils couldsuccessfully and economically be removed by the use of a burner in thepresence of a stream of oxygen-containing gas, without damage to thetubes. Such conditioning of the coke permitted the burner to be advancedthrough the tubes in a progressive manner and at a substantiallyconstant rate, while securing substantially complete removal of thecoke. This method of removing coke deposited in the treating of heavyoils has been found to possess marked advantages over previously knownmethods which employ mechanical devices. Not only are the tubes savedfrom mechanical abrasion and shock, but very substantial savings intime, materials and labor may be effected.

Obviously many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and therefore only such limitations should be imposedas are indicated in the appended claims.

I claim:

1. The method of reconditioning oil heater tubes containing cokedeposited therein during oil heating operations, which comprises passinga relatively clean and light oil through the tubes for a period ofseveral hours while continuing the heating operation, discontinuing theheating and while the tubes are hot steaming to remove oil and to drythe coke therein, then directing a localized flame in. progressivemanner through the tubes while simultaneously passing anoxygen-containing gas therethrough.

2. The method of reconditioning oil heater tubes containing cokedeposited therein during oil heating operations in which the oil heatedis a heavy oil, which comprises passing a relatively clean and light oilthrough the tubes for several hours while continuing the heatingoperation to displace the heavy oil from the tubes and to dissolve andremove heavy oil from the coke, discontinuing the heating and while thetubes are hot passing steam through the tubes for a period ,of severalhours to displace the light oil from the tubes and the coke therein,then directing a localized flame in progressive manner through the tubeswhile simultaneously passing an oxygen-containing gas therethrough.

CHESTER B. MCALLISTER.

